It is known that conventional monomode optical fibers used in optical communications systems exhibit birefringence characteristics varying both in time and along the fiber axis, so that the state of polarization of the propagating signals varies in a continuous and unpredictable manner. In coherent communications systems, and especially systems with heterodyne reception which are the most widely used, it is essential to have a definite and constant state of polarization at the fiber output. Indeed, the receivers can correctly operate only if the state of polarisation of the received signal coincides with that of the signal generated by the local oscillator; otherwise only part of the electromagnetic field undergoes heterodyning, with consequent signal fading which may even be total. The problem could be solved by using polarization maintaining fibers, that solution is seldom adopted since such fibers are rather expensive, have a high attenuation and may give rise to a noise increase if the alignment of the fiber axes at the joints, if any, is not correct. Thus, in coherent communications systems it is preferred to use conventional, low-birefringence optical fibers and to take measures for making the receiver polarisation-insensitive. This is generally achieved by scrambling the state of polarization of either the transmitted signal or the signal generated by the local oscillator in a non-adaptive continuous or discrete manner within each bit period.
Generally such scrambling is performed by means of modulators external to the source. Transmission rates of the order of some hundred Mbit/s or some Gbit/s are used in the coherent systems and the modulators allowing the state of polarization of the signals to be scrambled at a rate compatible with such bit rates are integrated-optics guide elements. Such elements are generally expensive and moreover give rise to high losses, due to both the attenuation of the guide itself and the coupling between the guide and the possible fiber piece allowing connection to an optical fiber.
In accordance with the invention, we provide a coherent optical communications system wherein polarization scrambling is obtained without using devices external to the source but by exploiting two orthogonally-polarised radiations at different frequencies.
The combination of two orthogonally-polarised radiations at different frequencies to obtain insensitivity to polarization fluctuations in detection systems or in optical fiber communications systems has been disclosed in the literature, yet the proposed systems exploiting this combination cannot be used to solve the problem of obtaining polarization scrambling without use of external modulators.
More particularly, the paper "Depolarized source for fiber-optic applications" presented by W. K. Burns et al. at the OFC'91 Conference (San Diego, Calif., Feb. 18-22, 1991) and published at page 205 of the conference proceedings, describes a system in which the two radiations, generated by respective lasers, are combined to obtain in line a depolarized radiation. The paper does not disclose how such depolarized radiation can be combined with information transmission in a coherent system, whereas the goal of the invention is (to provide a coherent system which does not require external devices to obtain polarization scrambling.) The two lasers emit always at a constant frequency, and the difference between the two frequencies is large compared to the frequencies encountered in the application of interest. If such a system was employed to obtain polarization scrambling in a coherent communications system, there would be a risk that the beats with the signal generated by a local oscillator would be outside the detector band, so that demodulation of the information would be impossible. Moreover, polarization scrambling causes a broadening of the information signal band which increases with the polarization scrambling rate. Should the system disclosed in the paper be used, receivers with very wide band would have to be employed, thereby causing a great increase in the noise present in the received signal.
The article "New polarization-insensitive detection technique for coherent optical fibre heterodyne communications" by A. D. Kersey et al., Electronics Letters, Vol. 23, No. 18, Aug. 27, 1987, discloses the use of the two orthogonally-polarised radiations at different frequencies to obtain the output signal of the local oscillator in a polarization diversity heterodyne receiver for a coherent communications system. The two radiations are obtained from a single source, by means of a polarising beam splitter generating the two orthogonal polarizations and of an acousto-optic modulator placed downstream the beam splitter, on the path of only one of the two orthogonally-polarized radiations. Also in this case, the two radiations always have a constant frequency and hence the considerations already made for OFC'91 still hold. Besides, even assuming that both signals outgoing from the acousto-optic modulator are used, and not only the frequency shifted signal, the invention cannot be obtained, since only one of the two radiations would be frequency-modulated. In the method of polarization scrambling in an optical signal transmission system, according to the invention, optical signals are modulated by a binary digital information signal and undergo heterodyne coherent demodulation. Two sources of linearly polarized radiation are frequency modulated by the information signal, so that the two sources emit radiations at a first and a second frequency or at a third and a fourth frequency, respectively, in correspondence with the first and the second logic level of the information signal. The third and fourth frequencies differ from the first and the second frequencies by an amount at least equal to the inverse of the bit period in the binary signal. The states of polarization of the radiations emitted by the two sources are rendered orthogonal in correspondence with each bit of the information signal. The two orthogonally polarized radiations are combined in order to send on a transmission line, for each bit, a signal comprising two orthogonally-polarized components at the first and the third frequency and at the second and the fourth frequency, respectively, whereby a linear polarization is obtained whose plane rotates by at least 360.degree. in the bit period.
Moreover linearly polarized radiations at a first and a second frequency are generated in a local oscillator by means of two independent sources, the two frequencies differing by an amount at least equal to the inverse of the bit period in the binary signal. The states of polarization of the radiations emitted by the two sources are rendered orthogonal and the two orthogonally polarized radiations can be combined in order to generate a signal comprising two orthogonally-polarized components at the first and the second frequency, respectively, whereby a linear polarization is obtained with a plane of polarization rotating by at least 360.degree. in the bit period, this signal being combined with the received modulated signal.
In these systems the frequency difference can be equal to the inverse of the bit period.
The transmission system using polarization scrambling can comprise a transmitter with means for modulating an optical signal with a binary information signal and means for continuously scrambling, in each bit period, the state of polarization of the modulated signals. The system also includes a receiver comprising a local oscillator generating a signal which is combined with the signal. The means for sending the modulated signals on the line and for scrambling the state of polarization of such signals are made by the same group of devices and which can comprise:
a first source of light radiations, which emits radiations linearly polarized in a first plane and is biased by the information signal so as to emit radiations at a first or a second frequency according to whether the information signal presents the first or the second of the two logic levels;
a second source of light radiations, which emits radiations linearly polarized in a second plane orthogonal to the first plane and is biased by the information signal so as to emit radiations at a third or a fourth frequency, according to whether the information signal presents the first or the second of the two logic levels, the third and fourth frequencies being very close to the first and second frequencies but differing therefrom by an amount at least equal to the inverse of the bit period; and means for combining the radiations emitted by the two sources into a single radiation which is sent onto an optical fiber transmission line and comprises, for each of the two logic levels of the information signal, two orthogonally-polarized components having the first and third frequencies or the second and fourth frequencies, respectively, so that, for both logic levels, a resulting linear polarization is obtained the plane of which rotates by at least 360.degree. in the bit period.
Where the local oscillator is used, the means for scrambling the state of polarization of the signals generated by the local oscillator can comprise:
a first source of light radiations, which emits radiations linearly polarized in the first plane;
a second source of light radiations, which emits radiations at a second frequency linearly polarized in a second plane orthogonal to the first plane, the second frequency being very close to the first frequency but differing therefrom by an amount at least equal to the inverse of the bit period of the information signal; and
means for combining the radiations emitted by the two sources into a single radiation which comprises two orthogonally-polarized components and has a resulting linear polarization the plane of which rotates by at least 360.degree. in the bit period.